1. Technical Field
The present invention relates in general to the field of computers, and in particular to computers having peripheral devices attached to a motherboard in the computer. Still more particularly, the present invention relates to a method and system for the peripheral device to switch from an internal local clock signal to an external master clock signal that is generated by the motherboard.
2. Description of the Related Art
Modern electronic devices such as computers often have peripheral devices associated with them. One such peripheral device is a controller card, which is used to control operations of one or more motherboards. In order to be able to control the motherboards regardless of whether the motherboard is powered up or not, such controller cards often have an on-board controller chip.
FIG. 1 depicts a typical peripheral device, which is shown for exemplary purposes as a Peripheral Component Interconnect (PCI) card 102, which is compliant with the PCI Local Bus Specification published by the PCI Special Interest Group of Hillsboro, Oreg. PCI card 102 has a primary PCI bus 104 on which outbound data is sent, a secondary PCI bus 106 for receiving inbound data, and a PCI controller chip 108 that provides a PCI clock signal to both PCI busses (104 and 106) in PCI card 102. Note that without a PCI clock signal, PCI busses 104 and 106 are disabled, thus making it impossible to configure/use PCI card 102.
PCI card 102 is typically coupled via PCI slots to a motherboard. When the motherboard is turned off, the PCI card 102 is in a “stand alone” (autonomous) mode, having its own power and being controlled by its own PCI controller chip 108, shown in FIG. 1. However, when the motherboard, to which PCI card 102 is attached, is powered up, then the motherboard will typically assert control over the PCI card 102, including an assertion of a master clock signal. For example, as shown in FIG. 2, a motherboard 202 is coupled to PCI card 102 via a PCI slot 204 and a PCI connector 206. When motherboard 202 powers up, motherboard's PCI controller chip 208 sends a PCI clock signal (along with a synchronization reset signal) to all PCI devices coupled to motherboard 202, including PCI card 102. PCI card 102's PCI busses 104 and 106 then experience “clock collision.” That is, PCI busses 104 and 106 are receiving competing PCI clock signals (internally from PCI controller chip 108 and externally from PCI controller chip 208). This causes PCI card 102's busses to malfunction.